Disconformities and paleoceanography in the southeast Indian Ocean during the past 5.4 million years

Biostratigraphy and magnetostratigraphy are used to identify deep-sea disconformities in the southeast Indian Ocean. The hiatuses are used to define periods of increased bottom-water activity and to indicate deep-sea circulation patterns during the Pliocene and Pleistocene. Seven zones of hiatus are restricted to areas confined by topographic restrictions or by water-depth control on bottom-water flow. An analysis of hiatus frequency reveals that a water depth of about 4,200 m separates disconformities formed by Antarctic Bottom Water (>4,200 m) and Circumpolar Deep Water (< 4,200 m). Hiatus zones in deep basins occur in the Tasman, Emerald, Balleny, and South Indian Basins and east of the Macquarie Ridge. Shallow-water hiatus zones (< 4,200 m) occur at the junction of the Southeast Indian Rise, the Macquarie Ridge, and the Tasman Basin and along the Antarctic Margin.

Changes in the extent of these regional hiatuses are examined by analysis of hiatus occurrence in seven time slices during the past 5.4 m.y. Although the timing of hiatuses produced by deep and shallow water masses are not synchronous, the causes of increased bottom-water activity may be correlated with paleoclimatic events. Southern Ocean cooling and the proposed re-establishment of the West Antarctic ice sheet and its fringing ice shelves in the late Gilbert Chron (t ≃ 4.2 to 3.6 m.y. B.P.) were accompanied by increased Antarctic Bottom Water and Circumpolar Deep Water activity as revealed by the frequency of both shallow and deep disconformities. The formation of the Northern Hemisphere Barents Ice Sheet at ∼3.5 m.y. B.P., however, led to a decrease in Antarctic Bottom Water production by inhibiting the formation of saline North Atlantic Deep Water, which combines with cold Antarctic shelf water to form Antarctic Bottom Water. Cool Southern Ocean climatic conditions that culminated in the late Matuyama Chron (t = 1.66 to 0.72 m.y. B.P.) resulted in a widespread disconformity produced by increased Circumpolar Deep Water activity and a more restricted disconformity produced by Antarctic Bottom Water. The areal extent of both deep and shallow disconformities was reduced after the late Matuyama Chron (t⩽0.72 m.y. B.P.) in response to decreased bottom-current activity.